Direct towards special fluid property of AZ91D magnesium alloy and non-transparency of metal casting mould,we simulate filling process of four kinds of die casting gating and runner system,which we have designed with finite difference simulation software FLOW3D to predict the location and size of surface defects,so that we can find the best gating and runner system.The result shows under the condition of appropriate parameters:pouring temperature 630 ℃,mold temperature 220 ℃,ingate velocity 60m/s,the design is rational when the liquid metal uniformly flow over the part,and on the basis of optimum design,we can produce the magnesium part with dense microstructure.